WATERLOO, N.Y. — An upstate New York man admitted Wednesday that he crushed his son under a truck in a life insurance scheme that netted him $700,000, authorities said.

Karl Karlsen’s plea of guilty to second-degree murder came as a surprise a day before his trial was set to open, Seneca County District Attorney Barry Porsch said.

Karlsen, 53, was charged with murder and insurance fraud in the death of his son Levi, 23. Prosecutors said Karlsen killed his son in 2008 by shifting a truck off its jacks as the younger man worked underneath it in a barn on the family’s property in Romulus, 40 miles southwest of Syracuse.

The sentence Karlsen faces on Dec. 16 wasn’t released. Porsch said more details about the plea will be released Thursday.

Authorities said Levi Karlsen’s handwritten will left everything to his father and was notarized at a bank on the morning of Nov. 20, 2008, days before his death.

After he died, his father told sheriff’s deputies he had returned from a funeral and found the truck had fallen off a jack and trapped his son.

Karlsen was arrested last year after investigators learned about the life insurance policy.

They said Karlsen gave differing accounts of his son’s death during almost 10 hours of interrogation, saying at one point it was an accident that happened after Levi asked him to move the jacked-up truck and at another saying he found him dead before going to the funeral. He said in each case he didn’t try to get help.

His second wife, Cindy Karlsen, testified during a pretrial hearing that she began to suspect in the summer of 2011 that her husband had killed his son.

She said she learned he had used proceeds from Levi’s life insurance settlement to buy a policy on her.

“I found out it was actually a life insurance policy on me and I would be worth $1.2 million dead to Karl,” Cindy Karlsen said.

Investigators have also been reviewing the 1991 death of Karlsen’s former wife, Christina Karlsen. She died in a Calaveras County, Calif., fire that also resulted in an insurance payout. Karlsen told investigators at the time that he was able to rescue Levi and the boy’s sisters but could not save his wife. He collected $200,000 in insurance on his wife after the fire was declared an accident. Karlsen has denied causing her death.

The Calaveras County district attorney’s office said nobody was available to comment Wednesday.

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